Leg 149 was the first of a series of ODP legs designed to study the sedimentation history and
tectonic evolution of the Iberia-Newfoundland conjugate rifted margins. The west Iberia margin
comprises three segments which have experienced progressive breakup from south to north in
Early Cretaceous times. The Ocean-Continent Transition (OCT) in the central segment has been
located by seismic reflection and refraction profiles and by magnetic and gravity modeling which
indicated that the oceanic crust adjacent to the OCT is thin, of the order of 4 km. Geophysical data
suggested that seafloor exposures of mantle peridotite on the western margin of Galicia Bank
extend southward beneath the sediments of the Iberia Abyssal Plain.

Drilling was conducted at five sites (Site 897 to Site 901) along a transect of the OCT, the main
sites located over basement highs. The location of a peridotite ridge marking the OCT beneath the
sediments of the Iberia Abyssal Plain was correctly predicted, but the discovery an outcrop of
peridotite, possibly 19 km wide, was completely unexpected. Apparently, the peridotites were
brought up to the seabed by the final stretching and break-up of continental crust as Newfoundland
separated from Iberia about 130 Ma. One basement high was capped by a set of three unusual
breccia flow units composed almost entirely of serpentinized peridotite and underlain by a mass-
flow deposit. These breccia units have the characteristics of rapidly deposited flows with shear
deformation under low normal stress typical of high fluid pressures. The unusual character of these
flows, and their rare occurrence in the oceans, makes it difficult to establish the mechanism of their
deposition. The mass-flow deposit was cored close to the top of the highest known basement high
in the area and, since mass flows are driven by gravitational potential, it is hard to see how clasts
from tens of miles away reached their present position unless there was a different arrangement of
basement relief in the Early Cretaceous and the site, which is now a high, underwent significant
uplift after deposition of the mass flow. From the region predicted to be extended continental crust,
basement consists of mettagabbro which may be part of the continental crust that was rifted to form
the margin or may have been emplaced, uplifted, and exposed during rifting at 125 Ma.

At the most landward site, sediments of Tithonian (Late Jurassic) age were recovered. These
sediments, deposited about 20 m.y. before the onset of sea-floor spreading on this segment of the
margin, are syn- or pre-rift in origin and suggest that this site overlies thinned continental crust.